Merlin doesn't support physical providers with no keystore file

At Merlin.loadProperties, in order for the load method to be called, there must be a non null inputStream.
In case of physical keystore (like smart card), there is no keystore inputStream, the keystore is not loaded and later on we'll get an error when we try
to retrive a certificate from the keystore.

The solution is to allow not getting a keystore file and in this case just send a null inputStream to the load method (it works).

There is a workaround: creating a new class which extends Merlin and just override the loadProperties method (though it's not perfect cause we can't
set the loadCACerts property (it's private).